The Ride Together Die Together challenge requires me to kill "Tumbaa" and "Pimon" within 10 seconds of each other. There's extremely little info about this challenge on the internet (no videos, best I can tell) but they are apparently named bosses who do not always spawn, and if one spawns the other isn't always there. One is a Skag, the other is a Stalker.
They apparently spawn in the Wildlife Exploitation Preserve, not in the Natural Selection Annex ("creature slaughter dome"), so you don't need the DLC to reach it. What I do not know is where these bosses spawn or how common they are. I've been through the preserve at least 4-5 times and never noticed these bosses. Where are they and what do they look like?